I think the main problem Chris had with Becca was that she really hadn't ever committed to ANYTHING it life it seems. She basically didn't know what she wants out of life. I liked her a lot better that Whitney, but in some ways, while dim-witted isn't the word I'm looking for, I found her slower mentally & emotionally. She's never really been in a long-term committed relationship, she's never been in love, she works as a chiropractic assistant, which is the kind of career someone can switch into and out of. When Chris pressed her as to what she wants to do in life, she couldn't come up with any answers other than to give it a couple years. Since her life already has a pattern of being non-committal, they could end up two years down the road and STILL non-committal with her living in Chicago, STILL not knowing what she wants. She seems like she's kind of sleep walking through life.
Whitney, on the other hand, has a PLAN! She's had one looooong before Chris came into the picture. She knows what she wants. She already had her eggs frozen and has been planning ahead for quite some time now. Now, she's rearranged her career and seems to have some contingency plans in place with the sisters already.
She probably already has Chris' kitchen cabinets reorganized and everything alphabetized to her liking. She's a strong, type-A personality. To me, while she is certainly going to go after and get what she wants, she also knows she may have to compromise to get things to work. That's an old fashioned marriage value from previous generations, that isn't as popular in our quick, "in a microwave minute" current generation that tosses out things that get the slightest speed bump rather than doing what it takes to make them work. While the marriage may ultimately fail, I'd say after about 6-7 years, by then they will have 2-3 kids, and will really have given it all they've got to make it work. OR she & the kids will basically be spending the majority of their time in Chicago, (they go to a progressive school,) as their marriage drifted apart, but they do what they must to make it work for the kids.
In that sense, while Chris' heart may prefer someone softer like Becca or Britt, living on a farm, even if in more of a management position, he needs someone strong, more organized and knows what she wants. So Whitney fits better the kind of life and lifestyle he leads.